Nickel-iron electrolytes containing hydrolyzing metal ions and process of electro-depositing ferromagnetic films



April 14', 1970 Filed Sept. 18, 1967 Fe *coNc./ RELATIVE UNITS METAL DEPOSITED/CPS A. AMBROSIA ET AL 5 5 NICKEL-IRON ELECTROLYTES CONTAINING HYDROLYZING METAL IONS AND PROCESS OF ELECTRO-DEPOSITING FERROMAGNETIC FILMS z Sheets-Sheet 1 FIG.1

540- 10- Tm CONCENTRATION/ MOLES FIG.2

CONVENTIONAL BATH Al ADDIT|ON 1 l l l l 50 TIME/MINUTES -vENT0Rs ALPHONSE AMBROSIA HARALD DAHMS ATTORNEY April 14, 1970 A AMB RO SIA ETAL 3,506,547

NICKEL-IRON ELECTROLYTES CONTAINING HYDROLYZING METAL IONS AND PROCESS OF ELECTRO-DEPOSITING FERRQMAGNETIC FILMS Filed Sept. 18, 1967 a Sheets-Sheet 2 ollxla'ln CURRENT DENSITY/ M. AMP/(1M 4o- F|G.4

c 0 0 Fe 20- 0 l l l l l TIME/ secowos United States Patent 3,506,547 NICKEL-IRON ELECTROLYTES CONTAINING HYDROLYZING METAL IONS AND PROC- ESS OF ELECTRO-DEPOSITING FERROMAG- NETIC FILMS Alphonse Ambrosia, Mahopac, and Harald Dahms, Ossining, N.Y., assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed Sept. 18, 1967, Ser. No. 668,561 Int. Cl. C23b 5/32 US. Cl. 204-43 13 Claims ABSTRACT OF THE DISCLOSURE The method is one for electroplating magnetic films of nickel iron alloys using conventional plating baths in which there is added metallic ions having hydrolysis constants in a pH range of from pH 2 to pH 7. The method involves the preparing of an electrolytic bath comprising a nickel salt, an iron salt, an electrolyte and metal ions selected from the group consisting of aluminum, magnesium, and a rare earth metal. The substrate to be plated is immersed in the above bath and subjected to an electrolytic action. The plated film has uniform proportions of nickel and iron throughout the film thickness. Additionally, the plating bath is stable over a wide range of temperatures.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method of electroplating magnetic films and to an improved electroplating bath therefor.

Description of the prior art In the prior art, electroplating, because of its inherent simplicity, is used as a manufacturing technique for the fabrication of magnetic thin films. One of the severe problems in plating Ni-Fe magnetic films arises from the fact that when a plating current is initially applied to a Ni-Fe bath, the initial deposit is very rich in Fe content and thereafter decreases in Fe content until an equilibrium condition is reached, and the alloy having the desired proportion of nickel and iron is plated. This variance in the proportions of nickel and iron is produced within the first 500 A. to 1,000 A. of film deposited. Thus, when the final film is to have a thickness of 1,000 A. or less and the films are to be used in computer memories, which demand 0 constant magnetic characteristlcs across the entire film,

this initial iron-rich deposit becomes a severe problem. This is especially so in terms of the magnetostriction of the deposited film, since zero magnetostriction is achieved with alloys including approximately 80% nickel and iron. When the alloy varies by any considerable degree from these proportions, it does not exhibit zero magneto striction. Additionally, the prior art Ni-Fe plating baths are unstable especially when used at high temperatures. For example, such baths can be used only for about 1 to 2 hours, after which they become opaque due to the hydrolysis and oxidation of ferrous ions.

One solution to the problem is disclosed in copending application Ser. No. 573,417, filed in behalf of James M.

Brownlow on Aug. 18, 1966, and commonly assigned. In the method of this application a pulse plating technique is employed in combination with selective agitation of the bath between pulses to prepare films in which the proportion of the film which has a high iron content also has a high copper content so that it is not magnetic and, therefore, the magnetostriction problem is avoided.

Another solution to this problem is disclosed in copendice ing application Ser. No. 601,951, filed in behalf of James M. Brownlow and Harald Dahms on Dec. 15, 1966, and commonly assigned. This method employs shaped plating currents, either in the form of a continuous plating current or a series of shaped current pulses. Each such pulse, when initially applied, has a magnitude which is significantly greater than the magnitude required to plate the desired proportions of nickel and iron in the deposited alloy under equilibrium conditions. The current is thereafter reduced in time, and the shape of the current pulse is controlled so that the initial current of high magnitude is sufficient to cause more nickel to be deposited at that time and, thereafter, a sufiicient amount of nickel as the current is decreased, so that the proportions of nickel and iron remain essentially constant throughout the thickness of the film.

Other prior art attempts at obtaining electroplate Ni-Fe films in which the Fe concentration in the film is relatively constant, have been to initially maintain the Fe++ ion concentration at low levels in the plating bath. The prior art baths generally do not contain more than 10- mole/ liter of the Fe++ ions. Ferrous ions present in amounts greater than 10- mole usually result in large Fe concentration gradients in the plated film.

An article by C. Le Mehaut and E. Rocher beginning at p. 141 in the March 1965 issue of the IBM Research and Development Journal discusses a method of preparing zero magnetostrictive Ni-Fe alloy films in which Cu ions are added to the plating bath. The initial metal deposit is composed of Cu with the subsequent deposit being Ni-Fe and small codeposits of Cu. To obtain a magnetic film which is magnetostrictive by this method, it is necessary to carefully control the current density of the applied current.

Patent No. 1,960,029 to Alexander G. Russell discloses an electrodeposition method in which the ratio of the metals to be deposited in the alloy is maintained constant in solution by providing separate anodes of nickel and iron and adjusting the anode areas in the solution so that the current flows through the anode in the proper amount to replace the metal removed from the solution at the cathode.

In order to obtain Ni-Fe films in which the iron concentration is constant throughout the plating operation, by prior art methods, it is necessary to initially increase the Ni-Fe ratio, i.e., maintain a low concentration of ferrous ions, and to carefully maintain the diffusion conditions in the bath, i.e., the applied voltage and/or applied current. 5

SUMMARY OF THE INVENTION In the practice of the present invention, it is not necessary to use current pulsing nor is it necessary to closely control the concentration level of iron in the bath. Specifically, this method employs a conventional plating bath to which has been added metal ions exhibiting hydrolysis i.e., metal ions which form precipitable hydroxides, in the pH range of from pH 2 to pH 7 and having a deposition potential more negative than Fe and Ni, so that no electrodeposition of the additive occurs. Additionally, the prepared bath may be used for periods far in excess of the prior art plating bath without appreciable decomposition thereof.

It is therefore an object of the present invention to devise an improved method for the electrodeposition of a ferromagnetic coating onto an electrically conductive carrier, whereby magnetic memory elements are produced.

It is another object of the invention to devise an improved electroplating bath for utilization in the process of electrodeposition of a ferromagnetic coating onto a substrate.

The foregoing and other objects, features and advantages of this invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of nickel and iron deposition versus the concentration of a rare earth metal ion.

FIG. 2 is a plot of the ferric iron concentration variation with time for a conventional plating bath and for the plating bath of this invention.

FIG. 3 is a plot of the percentage of iron deposited versus current density.

FIG. 4 is a plot of the composition gradient versus time for the bath composition of this invention.

In accordance with a more specific aspect of the pres ent invention, the plating bath preferably contains simple salts of iron, nickel, and a metal selected from the group consisting of aluminum, magnesium, lanthanum, cerium, praseodymium, erbium, europium, gadolinium, hafnium, lutetium, neodymium, scandium, samarium, thulium, yttrium, ytterbium, holmium, terbium, and other rare earth metal ions hydrolyzing in a pH range of from pH 2 and pH 7. Iron is initially added as a ferrous salt, such as ferrous chloride (FeCl AH O), ferrous sulphate, or as other simple salts of iron. The ferrous salt is present in amounts sufficient to produce -Fe++ ions in concentrations of from to 0.5 mole per liter.

Nickel is preferably added to the plating bath as nickel chloride (NiCl .6H O). The nickel salt used is present in sufficient amounts to produce Ni++ ions in concentrations of from 0.1 to 0.5 mole per liter. However, following the addition, several nickel species may appear as free nickel ions, nickel amine complexes, nickel chelates, nickel addition agent complexes, and nickel complex with other added metal salts. Nickel may be added in the form of other salts, as an illustrative example, nickel sulphate, provided precipitation does not occur. The form in which nickel exists in a given system depends upon many factors, such as nickel concentration, pH, ammonium ion concentration, chelate agent concentration, iron concentration, addition agent concentration, complexing agent concentration, temperature, and concentration of other metals.

Aluminum, magnesium, and the above rare earth elements have an appreciable effect on the plating bath; and it is preferably added to the bath in the form of a simple salt. The choice of salts is dependent upon the ability of the given salt to dissolve in the plating bath, and that the metallic ion hydrolyzes before nickel and ferrous ions.

For example, the nitrates, chlorides, sulphates, etc. are salts that are found to be satisfactory. The rare earth metals are generally acquired as their sesquioxides, which are converted into one of the above mentioned salts. For example, the chloride salt of the rare earth metal is prepared by dissolving the sesquioxide in hot concentrated HCl, which is subsequently evaporated. The residue are crystals of the rare earth metal chloride. The above salts are added to the bath in amounts of from 7 l0- to 1X10 mole per liter. The pH of the bath is limited to a range of 1.5 to 7. Additionally, other additives, such as boric acid and saccharin may also be added to the bath.

The following examples are illustrative of the present invention. Controlled experiments are performed to better illustrate the significant advantages of this invention.

Example I Several plating baths were prepared having the following composition: 0.2 mole of NiSO 0.2 mole of FeSo 10 gms. of H BO /liter and 2 10- mole of H 80 in one liter of water. Thulium chloride was added to each of the severally prepared baths in amounts sufficient to provide Tinions in the range of 1 10- to 1 10' A current having a current density of ma./

.4 cm. was passed through the individual baths for one minute. A bright N-Fe film was deposited from each bath. The metal deposition was monitored by X-ray fluorescence to determine the amount of Ni and Fe present in each of the plated films. The amount of metal deposition is directly proportional to counts per second of the X-ray fluorescence.

It is seen in FIG. 1 that as the concentration of Tm+ ions is increased to a range of from 5 X 10- to 10- mole/ liter, the Fe content of the plated films decreased rapidly, after which a relatively constant Fe concentration gradient is evidenced. It is also observed that the Ni concentration gradient remained constant throughout the plating operations.

EXAMPLE II The experiments of Example I were repeated except that Sc+++ ions were used in place of the Tm+++ ions in the same concentrations indicated. A curve of the results obtained was plotted, which closely approximated that shown in FIG. 1.

EXAMPLE III The experiments of Example I were repeated except that Lu+++ ions were used in place of the T m+++ ions in the same concentrations indicated. A curve of the results obtained was plotted. The curve showed similar results to the curve shown in FIG. 1.

EXAMPLE IV In an effort to establish the stabilizing effect of Al ions on a plating bath, the following experiment was performed. A bath including 0.4 mole of nickel sulphate, 0.1 mole of ferrous sulphate, 10 gms. of H BO 1., 10 mole of H 50 in one liter of water was prepared. To one-half of the above prepared bath (bath 1) an amount of Al (NO was added such as to make the aluminum concentration 10- M. To the other half of the bath (bath 2) sulfuric acid was added to a concentration of 1.5.10 M H Nitrogen was continuously bubbled through the baths which were closed off from the atmosphere. The baths were then heated to a temperature of about C. and allowed to stand for about 3 hours. Aliquots of the baths were taken at timed intervals and the Fe+++ ion concentration determined. The Fe+++ ion concentration is plotted as a function of time to determine the rate of hydrolysis of the Fe ions.

Shown in FIG. 2 is the effect of the presence or absence of the Al ion on the rate of hydrolysis and oxidation of the Fe++ ions in baths 1 and 2. The curve represented by the triangular points (bath 1) shows that the hydrolysis and oxidation of ferrous ions in the presence of aluminum ions proceeds at a much lower rate when compared to the representative curve of the conventional bath 2 (circular points). This effect is not simply due to the lowering of the pH, since equivalent additions of sulfuric acid to bath 1 do not produce a similar effect.

EXAMPLE V A substrate was immersed in a plating bath consisting of 0.4 mole of nickel sulphate, 0.2 mole of ferrous sulphate, 10 grams of H BO 10 mole of H SO 2X10- mole of Al(NO and one gram of Na-saccharine per liter. The plating bath was heated to a temperature of about 95 C. Nitrogen was continuously bubbled into the solution. A current having a varying current density from 15 to 35 milliamperes/centimeters was applied. The resulting films had a bright appearance. It was found that the plating bath remained completely free of any iron oxide deposits for 3 /2 hours. An identically prepared bath, without the presence of aluminum ions and to which a current having a current density of 15-35 ma./cm. was applied, had formed an iron oxide deposit of from 0.05 to 0.10 inch at the bottom of the plating vessel after only 0.5 hour.

It is shown in FIG. 3 that the composition of the plated filn1 varies from 15% to 31% Fe. This range of Fe composition can be plated from a conventional bath, only it the bath contains an Fe concentration as low as 1X10 mole. This experiment demonstrates that with the addition of Al the iron content of the bath can be maintained at a much higher level and that the iron concentration of the bath does not have to be controlled as closely as in the case of plating from a conventional bath.

FIG. 4 shows that the iron composition gradients, which are commonly found in conventional constant current plating, do not occur in high temperature plating with the presence of aluminum, "magnesium, or a rare earth metal ion, i.e., the iron composition deposited from the baths of this invention remains relatively constant throughout the plating operation.

EXAMPLE VI The experiment of Example V was repeated except that 0.2 mole of MgSo .7H O was substituted for the Results similar to those of Example V were obtained.

Salts of cerium, erbium, europium, gadolinium, hafnium, lanthanum, neodymium, samarium, terbium, yttrium, holmium, praseodymium, dysprosium and ytter bium can be added in similar baths given in Examples I to V1 in amounts such that that metal ion was present in a concentration range of from 1 l0- to 1 10 mole per liter. Since all metal ion additions had similar effects on plate composition and bath stability, it is thought that the hydrolysis of the added metal ions is mainly responsible for the effects observed.

What is claimed is:

1. An aqueous electrolytic bath for use in the process of deposition of a ferromagnetic coating on an electrical- 1y conductive substrate, said bath having a pH in the range of from 1.3 to 7 and including as essential constituents ferrous ions in a concentration in the range of 1X10 to 5 10- mole/1., Ni++ ions in a concentration in the range of 1X10 to 5 10- mole/l. and metallic ions having negative deposition potentials such that said metallic ions do not codeposit from said bath with said Ni++ and said Fe++ ions, said metallic ions being selected from the group consisting of Al, Ce, Er, Eu, Gd, Hf, La, Lu, Nd, Sc, Sm, Tm, Tb, Y, Yb, Dy, Pr and Ho, and being present in the range of about 1 10- to 1X 10- mole per liter.

2. An aqueous electrolytic bath according to claim 1 wherein there is added 10 grams per liter of H BO 3. An aqueous electrolytic bath according to claim 1 wherein the metallic ions are Tm+ ions.

4. An aqueous electrolytic bath according to claim 3 wherein said metallic ions are Al+ ions.

5. An aqueous electrolytic bath according to claim 1 wherein said metallic ions are Sc+++ ions.

6. An aqueous electrolytic bath according to claim 1 wherein said metallic ions are Lu+++ ions.

7. A process for depositing a ferromagnetic thin film comprising the steps of:

(a) providing an aqueous electrolytic bath having a pH in the range of 1.3 to 7 and including as essential constituents F'E ions in a concentration in the range of IX l0 to 5 l0- mole per liter, Ni++ ions in a concentration in the range of 1X10 to 5X10 mole per liter and metallic ion additives having negative disposition potentials such that said metallic ion additives do not codeposit with said Fe++ and Ni++ ions, said metallic ions being selected from the group consisting of Al, Ce, Er, Eu, Gd, Hf, La, Lu, Nd, Sc, Sm, Tm, Tb, Y, Yb, Pr, Dy and Ho and being present in the range of about 1 10- to 1 10- mole/li ter.

(b) subjecting an electrically conductive substrate to an electrolytic action in said bath for a time sufiicient to effect the deposition thereon of a ferromagnetic film which has a uniform Fe concentration throughout its thickness.

8. A process according to claim 7 wherein there is added to said bath about 10 grams per liter of H 30 9. A process according to claim 7 wherein said metallic ions are Tm+ ions.

10. A process according to claim 7 wherein said metallic ions are Al+ ions.

11. A process according to claim 7 wherein said metallic ions are Sc+ ions.

12. A process according to claim 7 wherein said metallic ions are Lu+ ions.

13. A process according to claim 7 wherein there is added to said bath 1 gram per liter of Na-saccharin.

References Cited UNITED STATES PATENTS 1,837,355 12/1931 Burns et al 204--43 XR 1,960,029 5/ 1934 Russell 20443 3,047,475 7/ 1962 Hespenheide 204-43 3,138,785 6/1964 Chapman et a1 340174 3,255,033 6/ 1966 Schmeckenbecher 1061 XR 3,271,274 9/ 1966 Guilio et a1. 204-43 OTHER REFERENCES Abner Brenner: Electrodeposition of Alloys, vol II, pp. 243-245 and 265-269 (1963).

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. 01. X.R. 204-44 

